Industrially usable plastics generally contain a number of additives to improve their application properties, for example plasticizers, fillers, stabilizers, antioxidants, lubricants, release agents, antistats, colorants and further additives.
Frequently, it is found that undesirable interactions develop especially between colorants on the one hand and stabilizers and antioxidants on the other, presumably because the stabilizer and/or antioxidant molecules diffuse to the surface of the pigment particles and there lead to a yellowness reaction which frequently proceeds without exposure to light. This yellowness reaction leads, in particular in the case of light-colored pigments, to unattractive effects and appreciably impairs the aesthetics of the plastics system.
The prior art proposals for avoiding the yellowness reaction do lead to an improved yellowness stability, but do not succeed in completely suppressing the yellowing of plastics. For instance, EP 0 492 223 describes a process in which the reactivity of the TiO.sub.2 is reduced by silanizing the pigment surface. Also, the application of further metal oxide layers has been employed, but has the disadvantage, in the case of the use of colored oxide layers, of completely changing the color properties of the underlying TiO.sub.2 -coated pigment and, what is more, the metal oxide layer which then comes to lie on top frequently is just as highly surface-reactive and it may yellow.
The treatment disclosed in EP 0 520 313 A2 of coating the TiO.sub.2 layer with an alkaline earth metal titanate and subsequently calcining also does not lead to complete suppression of the yellowness reaction of pigmented plastics.